For insulating cups made from base paper as a base material, several structures provided with good touching feeling and insulation by foaming a laminate film laminated on the base material have been proposed.
The JP-A-57-110439, for example, discloses such a structure that a water content contained in paper is vaporized by heating and this vaporized water content is used as a foaming material so as to foam a thermoplastic synthetic resin such as polyethylene film.
With this method, however, the foaming height is limited, and there is a problem that, even if time for heating is prolonged, the foaming height can not be increased.
On the other hand, in the JP-A-61-246041, a foam sheet and a paper board in which a large number of small holes for ventilation are penetrated is integrally joined with a large number of partial adhesives so as to form a complex sheet, and by vacuum-suctioning this foam sheet after heated and softened at a position other than the partial adhesives and from the side of the foam sheet to inflate and form the foam sheet along a forming die so that swollen projections are formed on the foam sheet.
In this case, it is so constituted that the softened foam sheet is suctioned at a position other than the partial adhesion parts for expansion, which takes time and labor and causes the swollen portion to be largely expanded. That constitution is not suitable for a cup, and it is difficult to form a foaming plane with good feeling.
Also, the applicant proposed another foaming structure in the JP-A-2000-177039 that a joining inhibitor agent such as silicone oil is applied in spots on a paper board, on which a laminate film is joined, and the surface joined with said laminate film is heated and then, portions on the laminate film where the joining inhibitor agent was applied is swollen by vacuum-suctioning. This structure has achieved a due success.
In this structure, since the joining inhibitor agent is applied in spots on the paper board and the laminate film which was made plastic by heating is extended on said applied portions by vacuum-suctioning so as to swell and form a foamed portion, the foamed portions are arranged in the scattered manner and a continuous foaming plane without a gap can not be obtained.
The inventors have, after a study in view of the above, developed a structure a foaming plane with continuing foaming cells without a gap by foaming a synthetic resin film with moisture in base paper as a foaming agent and the foaming height is gradually increased by vacuum-suctioning this foaming plane.
That is, as with the above moisture foaming in JP-A-57-110439, the moisture contained in the base paper is vaporized by heating, and this vaporized moisture is used as a foaming material to foam a thermoplastic synthetic resin such as polyethylene film.
The synthetic resin film forming this foaming plane is preferably polyethylene, but a thermoplastic film such as polypropylene, polyvinylchloride, polystyrene, polyester and nylon is used.
The base paper with the basis weight of 100 g/m2 to 400 g/m2 containing about 2% to 10%, preferably 4.5% to 8% of water is used.
Heating temperature for foaming is generally in a range of about 110 to 200 degrees centigrade, but it is determined as appropriate by type of the base paper and thermoplastic resin film to be used.
According to experiments, in the case of a low-density polyethylene, an expansion ratio of 8 times was obtained with the heating temperature of 135 degrees centigrade and the heating time of 90 seconds. It was also confirmed that the foaming height can be increased to about 15 times when the heating time is 5 minutes.
When the heating temperature is raised, the water content in the base paper is decreased, but the expansion ratio was lowered.
This might be because the temperature of the resin largely exceeds a melting point, whereby foaming cells are broken, and the strength of the resin is lowered and the foaming cells shrink when cooled.
When the moisture in the base paper is used in this way, the expansion ratio can be temporarily increased but there is a critical problem that the foaming cells shrink at cooling and the foaming height can not be maintained.
In connection with that, in order to prevent contraction of the foaming cells at cooling, there can be a method that the strength of the foaming cells is reinforced by rapid cooling.
Then, water cooling and rapid cooling at −5 degrees centigrade were performed, but the foaming height can not be kept high.
Next, a method that a negative pressure is applied to the foaming plane so as to restrain contraction of the foaming cells as well as that the foaming cells are heightened by vacuum-suctioning was tried, and it was found that the height of the foaming plane can be maintained, which lead to successful completion of the present invention.
That is, a main object of the present invention is to provide a thermal insulation foamed sheet by forming a foaming plane made of a foamed cell group through foaming of a synthetic resin film mainly using moisture contained in base paper and by keeping the foaming height of this foaming plane by vacuum-suctioning, a thermal insulation foamed container using this and method for producing the same.